Software Engineering for Students A Programming Approach

246 Chapter 17 ■ Software robustness
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public void actionPerformed(ActionEvent event) {
String string = ageField.getText();
age = Integer.parseInt(string);
if (age > 18)
response.setText("you can vote");
else
response.setText("you cannot vote");
}
This piece of program, as written, provides no exception handling. It assumes that
nothing will go wrong. So if the user enters something that is not a valid integer,
method parseInt will fail. In this eventuality, the program needs to display an error
message and solicit new data, (see Figure 17.2).
To the programmer, checking for erroneous data is additional work, a nuisance, that
detracts from the central purpose of the program. For the user of the program, however,
it is important that the program carries out vigilant checking of the data and when
appropriate displays an informative error message and clear instructions as to how to
proceed. What exception handling allows the programmer to do is to show clearly what
is normal processing and what is exceptional processing.
Here is the same piece of program, but now written using exception handling. In
the terminology of exception handling, the program first makes a try to carry out some
action. If something goes wrong, an exception is thrown by a piece of program that
detects an error. Next the program catches the exception and deals with it.
public void actionPerformed(ActionEvent event) {
String string = ageField.getText();
try {
age = Integer.parseInt(string);
}
catch (NumberFormatException e){
response.setText("error. Please re-enter number");
return;
}
if (age > 18)
response.setText("you can vote");
else
response.setText("you cannot vote");
}
In the example, the program carries out a try operation, enclosing the section of program
that is being attempted. Should the method parseInt detect an error, it throws
a NumberFormatException exception. When this happens, the section of program
enclosed by the catch keyword is executed. As shown, this displays an error message
to the user of the program.
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